According to a conventional art for increasing the width of steel slabs during continuous casting, pouring of molten steel into a casting mold is stopped and after the molten steel is drawn from the mold as a cast slab, one or both side (shorter) members composing the mold is moved then a dummy bar having a width corresponding to the increased width is put in the bottom of the mold and the molten steel is poured again into the mold to obtain steel slab having increased or decreased width.
Therefore the conventional art has defects such that the casting operation must be completely stopped each time for changing the slab width, and a considerable time is required before the restoration of casting operation, thus causing remarkable lowering of continuous casting productivity.